Which is why it must be important to work out whether it's manmade or not. If it is natural, it must have been there when the tomb was erected. Erosion of granite takes much, much longer than erosion of, say, a hole in a limestone-built chambered tomb would.

In which case, the hole wasn't "pummelled" through the capstone, thus leading to the tomb's partial collapse.

For that to happen the stone would have to have been lying at the angle it is now either in the ground or above it and have a water 'trapping' area above it (which it hasn't) because the hole is perpendicular to the slope.